(1) Field of the Invention
The present invention relates to an image forming apparatus which forms images on the recording medium by causing the developer to jump thereto and can be applied to a printer unit in digital copiers and facsimile machines as well as to digital printers, plotters, etc.
(2) Description of the Prior Art
In recent years, as the image forming means for outputting a visual image on recording medium such as recording paper etc., in response to an image signal, image forming apparatuses have been disclosed in Japanese Patent Application Laid-Open Hei 4 No. 269,563, Japanese Patent Application Laid-Open Hei 6 No. 286,203 and Japanese Patent Application Laid-Open Hei 8 No. 99,433, for example, wherein charged particles are placed in an electric field so that they will jump by electric force to adhere to the recording medium whilst the potential to be applied to the control electrode having a number of passage holes located in the jump passage is being varied, to thereby form a latent image on the recording medium, directly.
In the above prior art, the aforementioned control electrode uses a configuration which has a plurality of electrodes and feeder lines and has an electrode having a function of shielding the electrical influence from the electrodes and feeder line from the toner support, or a configuration in which jumping toner is controlled based on matrix control.
FIG. 1 is a schematic diagram showing main components of a conventional image forming apparatus. This apparatus includes an image forming unit 1 having a toner supplying section 2 and a printing section 3. Toner supplying section 2 in image forming unit 1 is composed of a toner storage tank 20 for storing toner 21 as the developer, a toner support 22 of a cylindrical sleeve for magnetically supporting toner 21, a doctor blade 23 which is provided inside toner storage tank 20 to electrify toner 21 and regulate the thickness of the toner layer carried on the peripheral surface of toner support 22. Doctor blade 23 is placed on the upstream side with respect to the rotational direction of toner support 22.
Toner support 22 rotates in the direction of arrow A in the figure. Instead of supporting toner 21 bymagnetic force, toner support 22 is configured so as to support the toner by electric force or combination of electric and magnetic forces. Toner 21 supported on the peripheral surface of toner support 22 is made to stand up in `spikes` at the area on the peripheral surface facing control electrode 26.
Printing section 3 in image forming unit 1 includes: an opposing electrode 25 facing the peripheral surface of toner support 22; a high-voltage power source 30 for supplying a high voltage to opposing electrode 25; a control electrode 26 provided between opposing electrode 25 and toner support 22; a charge erasing brush 28; a charging brush 8 for charging a sheet of paper 5; a dielectric belt 24; support members 16a and 16b for supporting dielectric belt 24; and a cleaner blade 19. Applied between opposing electrode 25 and toner support 22 is a high voltage which produces an electric field needed to make toner 21 carried on toner support 22 jump toward opposing electrode 25.
Control electrode 26 is disposed in parallel to the tangent plane of the surface of opposing electrode 25 and spreads two-dimensionally facing opposing electrode 25, and it has a structure to permit the toner to pass therethrough from toner support 22 to opposing electrode 25. The electric field formed between toner support 22 and opposing electrode 25 varies depending on the potential being applied to control electrode 26, so that the jumping of toner 21 from toner support 22 to opposing electrode 25 is controlled.
Control electrode 26 is composed of an insulative board 26a, a high voltage driver (not shown), independent annular conductors, i.e., annular electrodes 27 and a shield electrode 39. Board 26a has holes forming gates 29, to be mentioned later, formed therein. Annular electrodes 27 are formed of copper foil, for instance, and are arranged around the individual holes in a predetermined layout. Each opening of the annular electrode forms a passage for toner 21 to jump from toner support 22 to opposing electrode 25. Hereinbelow, this passage will be termed gate 29. Shield electrode 39 is also formed of copper foil with an insulative layer 26b on the surface thereof, and is disposed on the toner support 22 side with respect to insulative board 26a. Configurations having such a shield electrode are disclosed in Japanese Patent Application Laid-Open Hei 4 No. 269,563 and Japanese Patent Application Laid-Open Hei 6 No. 286,203. Japanese Patent Application Laid-Open Hei 8 No. 99,433 disclosed a configuration in which jumping of toner is controlled by the control electrode which is driven by matrix control.
The aforementioned shield electrode 39 is provided to prevent toner 21 adhering to control electrode 26. Unless control electrode 26 has this shield electrode 39, it is impossible to avoid adherence of toner 21 to control electrode 26. If adherence of toner 21 to control electrode 26 occurs, the following defects arise.
In order to illustrate this situation, a control 26 without any shield electrode 39 is illustrated in FIG. 2, which is a sectional view of a control electrode having no shield electrode. First, a voltage which inhibits toner 21 from jumping (to be referred to hereinbelow as the OFF potential) is supplied to annular electrodes 27. In this state, when toner 21 is made to jump to gates 29, a voltage which causes toner 21 to jump (to be referred to hereinbelow as the ON potential) is applied to make toner 21 jump. In this case, as shown in FIG. 3, toner 21 jumps; some toner 21a passes through gates 29, other toner 21b jumps to areas other than gates 29, i.e., toward the surface of control electrode 26.
Normally, this toner 21b will return to toner support 22 when annular electrodes 27 are set at the OFF potential, but some of it, i.e., toner 21c as shown in FIG. 4 remains adhering to the control electrode 26. If toner 21c adheres to control electrode 26, the apparent potential of control electrode 26 relative to that of toner 21 on toner support 22 varies due to the charge on toner 21c. Illustratively, the potential of control electrode 26 tends to vary in such a way as to become close to the voltage for inhibiting toner 21 from jumping, thus making it difficult for the toner to jump. Further, even if a voltage for causing the toner to jump is applied to control electrode 26, toner 21 on toner support 22 does not receive attraction from the electric field for jumping and the desired transfer of toner will not occur. In this case, the resulting image will not have correct density, presenting a dim, blurred state without contrast. In this condition, a desired reproduction of halftones cannot be obtained, making it difficult to form a correct image. Further, in the case of a color image forming apparatus, proper reproduction of colors cannot be obtained because proper amounts of toners cannot transfer.
Furthermore, if the situation of the toner adherence to control electrode 26 becomes worse, the toner jumping becomes more difficult, and finally in the worst case no toner will jump. This causes image defects and difficulty in reproducing color images in the case of a color image forming apparatus.
Besides, if adhering toner 21c has adhered to the gate interior, the gate will become clogged as toner 21c builds up, causing physical difficulty in toner jumping. In this state, no dots can be formed causing printing deficiency and/or image defects.
As above, adherence of toner 21c at the gates and their vicinity, directly causes the above deficiencies. On the other hand, if toner 21 adheres to the areas other than gates 29, the following defects occur. As shown in FIG. 4, toner 21c adheres to areas other than the gates, building up as adhering toner 21d as shown in FIG. 5. Illustratively, under the condition where some toner remains adhering to control electrode 26, when the voltage for causing toner 21 jump is applied to control electrode 26, toner 21 that has been newly supplied to toner support 22 facing gates 29, jumps therefrom against already adhering toner 21d or its vicinity, possibly touching adhering toner 21d or colliding against it. At that moment, if the cohesion between the toner particles is very strong, the toner particles form an aggregation, clumping and remaining on control electrode 26. Similarly, as toner 21 repeatedly transfers and adheres to the toner aggregations staying on control electrode 26, the aggregations finally build up covering gates 29 as shown in FIG. 6. In this case, the gates are clogged in the same manner as above causing the same deficiencies. A further buildup of adhering toner 21d reaching the layer of toner 21 carried on toner support 22 as shown toner 21e in FIG. 6, destroys the layer of toner 21.
This not only makes it difficult to control transfer of the toner to the gate 29 which is located downstream of toner 21e but also induces toner clog of the other gates 29.
These deficiencies occur as a result of transfer of toner 21 from areas other than that facing gates 29 to control electrode 26 when the ON potential is applied to the annular electrodes 27. Therefore, it is preferable that, at least, no electric field for jumping should be created in areas other than those facing gates 29. The simplest way of achieving this is a provision of an electrode plate as shield electrode 39 on control electrode 26 between annular electrodes 27 and toner support 22. In this arrangement, when shield electrode 39 is applied with a voltage which is opposite to the polarity of the toner or at least produces an electric field which is able to revert the toner back to toner support 22, in theory no toner will transfer to the areas other than gates 29 and their vicinity on control electrode 26. In case that some toner transfers to the periphery of gates 29, the electric field between shield electrode 39 and toner support 22 reverts the toner back to toner support 22, thus no aforementioned deficiency will occur.
However, the practical situation is that, even if shield electrode 39 is provided and is applied with a voltage of like polarity to the toner, adherence of toner can be improved to some degree, but not to a perfect level. Eventually, at some point, some kind of the aforementioned toner clog will occur, causing the above deficiencies.
One of the reasons is that there is some of toner 21 which has the opposite charge-characteristics (to be referred to as opposite charged toner) to that of the desired charge. When shield electrode 39 has a voltage of the same polarity as that of toner 21 applied so as to create an electric field which reverts the toner back to the toner support, the toner having normal charge-characteristics, as intended, will not jump and adhere to shield electrode 39. However, some toner having the opposite charge existing in the layer of toner 21 still jumps toward shield electrode 39 and adheres to shield electrode 39. This opposite charged toner is, in general, present at some percentage and is very little, but it will resultantly transfer and adhere to shield electrode 39 after a prolonged period of the image forming operation or other causes. Thus, this opposite charged toner will gradually build up with the passage of time, finally growing into toner aggregations like adhering toner 21d and 21e as stated above and consequently causing printing deficiencies and toner clog in gates 29 as stated above.
It is very difficult to produce toner absolutely free from opposite charged toner as long as normal toner is used. Even through a toner which is completely free from oppositely charged toner can be produced, its price will be extremely high, resulting in practical difficulties. Accordingly, the configuration in which toner 21 is placed in a neutral electric field is the most preferable. In the prior art disclosed in Japanese Patent Application Laid-Open Hei 4 No. 269,563, a reference electrode having the voltage applied to the sleeve (toner support) is used as the shield electrode (see FIG. 1).
However, in practice, the charge carried on toner 21 has a potential relative to sleeve 22, the surface potential of toner 21 supported on sleeve 22 has a potential of a like polarity to that of toner 21. This creates a potential difference between shield electrode 39 and the toner layer surface, forming an electric field, so that toner 21 on the sleeve, in particular the topmost surface of toner 21 is electrically attracted toward the shield electrode. This electric field is trivial. Nevertheless, even in this case toner will not be completely stopped from jumping by the electric field, resultantly a trace amount of toner will jump to shield electrode 39. In the case where a prolonged period of printing is repeatedly performed under this condition, the trace amount of toner 21 jumping by the electric field builds up, resultantly forming aggregations of toner 21 such as adhering toner 21d or 21e, thus causing toner clog in gates 29 and inducing printing deficiencies.
Further, in the case where the control electrode of the above prior art is used, the potential required for controlling the jump of toner 21 tends to become higher because annular electrode group 27 to which the voltage for controlling the toner jumping is applied is located more distant from toner support 22 than shield electrode 39. In general, the closer to toner support 22 electrode group 27 is located, the lower is the potential. As a result, the withstanding voltage of the transistors etc. used in the voltage switching means can be reduced further, facilitating the reduction of the cost of the switching circuit. In the above prior art, however, because of its configuration requirements, it is impossible to arrange annular electrode group 27 and shield electrode 39 on the same plane; this means that a higher voltage than that minimally required for controlling the toner jumping is needed, making it difficult to reduce the cost relating to the voltage switching means.
In a type of image forming apparatus of the above prior art, since the amount of toner that jumps is controlled by the electric field formed between gate 29 and toner support 22, the amount of toner that jumps will differ if the electric field is different. In the prior art stated above where toner support 22 of a cylindrical sleeve and control electrode 26 having a two-dimensional gate array are used, the distance between toner support 22 and control electrode 26 is not uniform due to the curvature of the sleeve. At the side areas of support 22, its distance from the control electrode is greater than from the central portion. Accordingly, the electric field at areas to the side is weak, so that the amount of toner passing through gates 29 and the track of the passage of toner are not uniform, resulting in dots thin in contrast at areas to the side and thick dots at the central area. As countermeasures against this, some techniques have been used such as increasing the voltage to be applied to the electrode at areas to the side when toner passes through.
However, the configuration in which the voltage for controlling the toner jumping is adjusted not only needs an increased number of power sources but, also needs extra high withstanding voltage FETs if the potential difference exceeds the nominal withstanding voltage of the current FETs used for the voltage switching means. This necessitates high withstanding voltage insulation for the circuits and increase in the cost of the FETs, needing more parts and unavoidably resulting in increase in size and cost of the apparatus. If the toner control of jumping is performed without increasing the withstanding voltage of FETs, the following deficiencies occur.
If the control voltage for toner jumping is increased without increasing the withstanding voltage of FETs, either the potential to be applied for making the toner jump (to be referred to hereinbelow as the ON potential) or the potential to be applied for prohibiting the toner from jumping (to be referred to hereinbelow as OFF potential) must be lowered. If the OFF potential is increased, the ON potential must be decreased, resulting in insufficiency of toner transfer and hence producing a blurred image without contrast. On the other hand, the ON potential is set higher, the OFF potential must be reduced. In this case, the stoppage of toner jumping cannot be correctly achieved, causing background fogginess, producing an image without contrast and thus making it difficult to achieve a satisfactory image forming operation. In the case of a color image forming apparatus, desired toner jumping cannot be obtained causing image degradation with insufficient reproduction of colors.
To deal with this, an attempt for varying the size of the electrode has been attained as in Japanese Patent Application Laid-Open Hei 8 No. 99,433. In this conventional art, the toner supported on the toner support jumps to areas other than the gates on the other electrode of the control electrode. Most of the toner having transferred to the control electrode will return to the toner support when the potential of the control electrode is switched. However, there is some toner which stays on the control electrode as already stated, and the remaining toner causes the apparent voltage of the control electrode to vary, resulting in insufficiency of toner jumping. With a further increase of the toner adherence, the toner will finally cover the gates and build up to destroy the toner layer carried on the surface of the toner support.